18th Edition – Reminder RCD Testing for different Types

80% from site during test and inspection, the other 20% are from Engineers who are preparing / planning a job. Having a basic understanding of the RCD tripping parameters defined in the product standards, for different types of residual currents, can save time on site when testing unfamiliar devices.

RCD Characteristics / Clause 531.3.3

We have summarised the tripping characteristics for the various types of RCD detailed in 531.3.3 (see table further on). This can be used as a quick guide, for specific information for design purposes refer to the relevant product standard.

For example, a Type A RCCB (EN61008-1) when subjected to an AC sinusoidal residual current is designed to trip at >50%<100% I∆n. The limits for pulsating direct current are different e.g. at O° >35% < 140% I∆n.

We have come across examples of RCD Test Instruments that appear to have incorrect values programmed into the software. One example recently, testing on 30mA Type A setting ½ Pulse, recorded as fail - tripping at 12 mA i.e. 40%. The customer was not aware of the difference in the trip values, for AC and pulsating DC residual currents. He assumed the problem was associated with the RCD, not the software in the test instrument. If in doubt check with the Instrument Manufacture for software updates or your Trade Association, who may already be aware of any issues.

RCD Tests / Clause 643.8

The RCD verification test for a 30mA RCD (5IDn <40 ms) relates to an AC sinusoidal residual current, irrespective of the Type of RCD. If the RCD appears to be tripping out of characteristic, when carrying out the test, check that you have the correct setting selected.

RCD tripping characteristics – Examples RCCBs EN 61008, EN 62423

The standards define the tripping thresholds and times based on the residual current components (see table below). Note key differences between tripping thresholds for AC, Pulsed and Smooth DC.

Tripping threshold:

• All RCCBs subjected to 50Hz AC residual current are designed to trip between 0.5-1I∆n.
• Type A,F,B: Pulsed residual current (50Hz), the tripping threshold changes to 0.35-1.4I∆n.
• Type A & F: Trip threshold is influenced by the smooth DC content (offset value) e.g.

Type A must still trip with < 6mA smooth DC superimposed on the pulsed waveform.

Type F must still trip with < 10mA smooth DC superimposed on the pulsed waveform.

• Type B smooth DC residual current, are designed to trip between 0.5-2I∆n.

Tripping times:

RCDs for use by ordinary persons namely RCCBs (EN 61008) and RCBOs (EN 61009), have fixed time characteristics i.e. the above standards do not allow adjustable characteristics.  CBRs and MRCDs to EN 60947-2 Annex B & M can have adjustable characteristics, designed for use in installations under the supervision of Skilled Persons – See Regulation 411.4.4 Note 2.

Appendix 3 Table 3A of BS7671, RCCBs & RCBOs relates to tripping for AC residual currents

Terminology can be confusing; we get regular requests for 30 mA Timed Delayed /Selective RCDs. This is not possible for safety reasons. However, to reduce nuisance tripping in circuits  associated with LED lighting, inverters, EVs etc, you can use a 30 mA Short time delay RCD. These devices have a built in 10 ms delay , but still meet the tripping requirements <40 ms at 5I∆n. 

Further Information

The complexity of modern installations particularly those associated with special locations, due to the nature, characteristics and variation of the appliances, fixed loads and improved safety requirements, requires specific competences and knowledge. The IET resources web site gives access to a wide range of additional books and guidance for special locations. These publications contain additional information on RCD requirements, supporting the Wiring Regulations.